1. Field of the Invention
The present invention relates to a linear motion device which causes a linear motion by the utilization of the rolling of rolling elements, such as steel balls or ceramic balls. More particularly, the invention relates to improvements of a structure of the retaining pieces interposed between the adjacent rolling elements and a linear motion device using the retaining pieces thus structured.
2. Description of the Related Art
Examples of the known linear motion device utilizing the rolling of a number of rolling elements are a linear motion guide bearing such as a linear guide or a linear ball bearing, and various types or screws having balls. Those linear motion devices are widely used as important mechanical components in many industrial machines.
As an example or this type of the device, a linear guide 10 is illustrated in plan, partly cut out, in FIG. 9. As shown, the linear guide 10 generally includes a guide rail 1, which axially extends, and a slider 2, shaped like U in cross section, which straddles the guide rail. Ball rolling grooves 3, while axially extending, are respectively formed in both side surfaces of the guide rail 1. A ball rolling groove 5, which faces a ball rolling groove 3, is formed in the inner wall of each of both side portions of a slider body 2A of a slider 2. A number of steel balls B as rolling elements are put between those opposed ball rolling grooves 3 and 5. The slider 2 is axially movable on and along the guide rail 1, with the assistance of the rolling of the steel balls B. With the movement of the slider, the steel balls B, which are interposed between the guide rail 1 and the slider 2, will move to the end of the slider body 2A of the slider 2. To continuously move the slider 2 in the axial direction, an endless circulation of those steel balls B is needed.
A linear through-hole 6 is axially formed in each side portion of the slider body 2A. The linear through-hole serves as a ball return path. End caps 7 are applied to both ends of the slider body 2A. Ball circulation tracks 8, semicircular in cross section, are formed in the caps so as to interconnect a path defined by the ball rolling grooves 3 and 5 and the linear through-hole 6, whereby a ball endless circulation track 9 is formed.
A ball screws mechanism 20 is perspectively illustrated, partly cut out, in FIG. 10. As shown, a ball nut 12 is fit to an axially extending threaded shaft 11. A thread groove 11a, semicircular in cross section, is formed in the outer peripheral surface or the threaded shaft 11. A thread groove 12a, semicircular in cross section, is formed in the inner peripheral surface of the ball nut 12. The thread groove 11a of the threaded shaft 11 is confronted with the semicircular thread groove 12a of the ball nut 12. A number of steel balls B as rolling elements are put in a path defined by those thread grooves 11a and 12a. The threaded shaft 11 and the ball nut 12 are relatively moved in the axial direction with the assistance of the rolling of the steel balls (for example, the threaded shaft 11 is axially rotated while the ball nut 12 rectilinearly moves.). With the relative movement, the steel balls B move while rolling in and along the spiral track defined by the thread grooves 11a and 12a. To continuously move the ball nut 12, an endless circulation of those steel balls B is needed.
A couple of circulation holes 17 are formed in the thickness of the ball nut 12 in a state that it straddles the threaded shaft 11 and is opened to the outside of the nut. The circulation holes 17 communicatively connect to the spiral track while extending in the tangential direction with respect to the path. A ball circulating track 18 is formed by coupling the circulation holes 17 by means of a ball tube 14A shaped like U (two ball circulating tracks 18 are used in the illustrated instance). After the steel balls B move a distance of, for example, 1.5 turn within the spiral track, those are introduced into the ball circulating track 18 and move in an endless circulating manner.
Smoother and more stable operations of the steel balls B as rolling elements, low noise and the like are required for the linear motion device. To meet the requirements, a measurement has been employed in which a spacer ball of the reduced diameter is interposed between the adjacent load balls. Where the measure is employed, the following problem arises anew, however. As the result of the interposing of the spacer ball, a span between the load balls is elongated. As an inevitable consequence, the load capacity and rigidity of the linear motion device are reduced. Therefore, a following approach is experimentally made. Retaining pieces (also called separators) 30 are each interposed between the adjacent steel balls B as shown in FIG. 11. By the retaining pieces 30 thus arranged, gaps in the row of the steel balls in their moving direction are eliminated and compression force is forcibly plied to the steel balls B, whereby the operability and the noise characteristic are improved.
To further improve the operability and noise characteristic, various proposals have been made which are constructed giving attention to the shape of the retaining piece 30 and the gaps present between the adjacent steel balls B and the retaining pieces 30.
The applicant of the present patent application proposed the following retaining piece structures in Japanese Patent Unexamined Publication No. 2000-120825:
1) In the retaining piece structure, a retaining piece has two recessed surfaces respectively facing to the balls. Each recessed surface is shaped such that it contacts with the steel ball at its outer edge or near outer edge. With this novel and unique technical feature, the low noise generation is achieved, and the operability is improved by maximizing the overlapping width of the retaining piece over which it contacts with the steel ball. Further, the spans between the steel balls are controlled at high precision.
2) In another structure, a retaining piece has recessed surfaces so shaped as to be in liner contact with the balls. This retaining piece structure also achieves the low noise generation. Further, the operability is improved since sliding resistance between the retaining pieces and the balls is small, and stable ball holding by the retaining pieces is secured.
The applicant of the present patent application proposed novel and unique retaining piece structure in Japanese Patent Unexamined Publication No. 2000-213538. In the retaining piece structure, proper gaps are provided in one ball row containing retaining pieces and balls, whereby the low noise generation and the improvement of the operability are achieved (those effects are high in level when the gap dimension is selected to be within 2% to 63% of the ball diameter.).
The retaining piece structure of Japanese Patent Unexamined Publication No. 2000-120825 is capable of satisfying the proper gap dimension described in Japanese Patent Unexamined Publication No. 2000-213538 if it is used in normal condition and the gap dimension is within a proper range of dimension values. Under a large pre-load and a large moment load, a load acting on the retaining piece (a pressing force of the ball against the recessed surface of the retaining piece) is large. As a result, the retaining piece is deformed, the ball-to-ball span is reduced, and the ball row will possibly lose its proper gap dimension.
When the linear motion device is used for a long time under hard conditions, the recessed surfaces of the retaining pieces will be worn and/or yielded. In this case, the ball-to-ball span is considerably reduced, the proper gap dimension is lost, and the revolution of the ball is abnormal. This problem remains unsolved.
An explanatory figure of the above structures is FIG. 8. In this figure, by the retaining pieces 30A arranged, gaps in the row of the steel balls in their moving direction are eliminated and compression force is forcibly applied to the steel balls B, whereby the operability and the noise characteristic are improved. Further, the span between the adjacent balls is reduced to minimize the reduction of the load capacity and rigidity of the linear motion device.
Also, in the aforementioned Japanese Patent Publications, the retaining piece 30A as the cylindrical member having the concave surfaces on both sides to be in contact with the balls B, B is made of synthetic resin and manufactured by injection molding as shown in FIG. 8. In this Figure, a gate portion G is provided on the outer peripheral surface thereof.
In the case of the retaining pieces 30A manufactured in a state that the gate portion G is provided on the outer peripheral surface thereof, the reduction of the load capacity and rigidity may be lessened by reducing the thickness of the retaining pieces 30A as small as possible. However, a gate-cutting step is inevitably needed after the injection molding process. Therefore, the cost to manufacture is increased by its cost. Further, the gate remainder after the gate cutting and the parting line of the molded product take the form of fins. As a result, the operability and the noise characteristic of the retaining pieces will possibly be deteriorated.
The present invention has been made to solve the above problem, and has an first object to provide a retaining piece structure and a linear motion device using the retaining pieces thus structured in which even when a load acting on the retaining pieces is excessive and the recessed surfaces of the retaining pieces are worn and/or yielded, a variation of the ball-to-ball span is minimized, and hence excellent improvements of the operability and the noise characteristic are maintained for a long time even under various use conditions.
In addition, the present invention has been made to solve the above problems and has a second object to provide a structure of a retaining piece and a linear motion device using the retaining pieces thus structured in which improvement is made about the gate position and the like in injection molding retaining pieces, whereby the operability and the noise characteristic are reliably improved at low cost.
To achieve the above first object, there is provided a structure of a retaining piece which is interposed each between the adjacent balls of a row of rolling balls, and concave surfaces of both side surfaces of said retaining piece as viewed in the axial direction are in contact with the spherical surfaces of the balls. The retaining piece structure is characterized in that each said concave surface is configured such that as a pressing force acting on said concave surfaces increases, each said concave surface increases a contact area thereof with the ball.
According to a second aspect of the invention, there is provided a structure of a retaining piece which is interposed between the adjacent balls of a row of rolling balls, and concave surfaces of both side surfaces of the retaining piece as viewed in the axial direction are in contact with the spherical surfaces of the balls. The retaining piece structure is characterized in that each the concave surface is configured such that as the wearing and/or yielding of the contact portion of the concave surface with the ball progress, the concave surface increases a contact area thereof with the ball.
According to a third aspect of the invention, there is provided a structure of a retaining piece which is interposed between the adjacent balls of a row of rolling balls, and concave surfaces of both side surfaces of the retaining piece as viewed in the axial direction are in contact with the spherical surfaces of the balls. The retaining piece structure is characterized in that each the concave surface includes a linear contact portion to be in contact with the ball, which is located close to the outer peripheral edge thereof, and an approximate curved surface having the radius of curvature approximate to that of the ball, which is located on the inner side of the linear contact portion (claim 3).
The invention also provides a linear motion device provided with retaining pieces each having any of the retaining piece structures described above.
In the retaining piece structure of the invention, each of the concave surfaces of the retaining piece a linear contact portion to be in contact with the ball, and an approximate curved surface having the radius of curvature approximate to that of the ball. Normally, the ball linearly contacts with the retaining piece, while the approximate curved surface having the radius of curvature approximate to that of the ball does not contact with the ball with a slight gap therebetween.
When a load acting on the retaining piece increases to excess a load value, or when the concave surface of the retaining piece is worn and/or yielded, a preset gap quantity between the retaining piece and the ball is reduced, and a span between the adjacent balls is reduced. Then, the ball starts to contact with the approximate curved surface having the radius of curvature approximate to that of the ball in the concave surface of the retaining piece. As a result, a contact area of the ball B pith the approximate curved surface 102 increases, and no further reduction of the span L progresses.
Next, to achieve the above second object, a fourth to ninth aspect of the invention are provided as follows. According to a fourth aspect of the invention, there is provided the adjacent balls of a row of rolling balls, and concave surfaces of both side surfaces of the retaining piece as viewed in the axial direction are in contact with the spherical surfaces of the balls, the improvement comprising a gate portion used in a process of molding retaining pieces is provided in the concave surface.
According to a fifth aspect of the invention, in the retaining piece structure of the fourth aspect of the invention, a recess is provided at a joining portion joined to the gate portion used in the process of molding the retaining pieces.
According to a sixth aspect of the invention, there is provided a structure of a retaining piece which is interposed between the adjacent balls of a row of rolling balls, and concave surfaces of both side surfaces of the retaining piece as viewed in the axial direction are in contact with the spherical surfaces of the balls, the improvement comprising a holder portion which holds an ejector pin used in a process of molding the retaining pieces is positioned in the concave surface.
According to a seventh aspect of the invention, in the retaining piece structure, a recess is provided at a joining portion joined to the ejector pin used in the process of molding the retaining pieces.
According to an eighth aspect of the invention, there is provided a structure of a retaining piece which is interposed between the adjacent balls of a row of rolling balls, and concave surfaces of both side surfaces of the retaining piece as viewed in the axial direction are in contact with the spherical surfaces of the balls, the improvement comprising a parting line used in a process of molding the retaining pieces is set on the outer peripheral surface of the retaining piece, and a recess or a tapered portion is provided near the parting line on the outer peripheral surface.
According to a ninth aspect of the invention, there is provided a linear motion device provided with retaining pieces each having any of the retaining piece structures described above.
In the retaining piece structure of the invention, the gate portion and the ejector pin, which are used in the process of molding the retaining pieces, are provided in the concave surfaces of both side surfaces of the retaining piece as viewed in the axial direction. Thus, in the mold separation step after the molding process, the retaining piece is automatically separated from the gate. Therefore, the gate cutting process, which is essential to the conventional technique, is eliminated, and this leads to cost reduction.
A recess is provided at a joining portion of the concave surface to the gate portion or the ejector pin position or near the parting line. In addition, fins and the like formed in the molding process are out in the recess. Accordingly, it never happens that when the retaining pieces are incorporated into the linear motion device, the fins or the like come in contact with the balls of the linear motion device and the inside and outside guide members, to thereby deteriorate the operability and the noise characteristic.